The invention relates to improvement of an electric rotary machine and, more particularly, to an electric rotary machine free from local over-heating at the ends of its stator core.
In general, a generator, for example, is directly coupled through its a rotary shaft with the shaft of a prime-mover so that mechanical energy transmitted through the rotary shaft from the prime-mover is converted into electrical energy. The prime-mover for a large capacity generator in a heat power plant, for example, generally employs a steam turbine. In order to convert the heat energy of steam at high pressure and temperature into mechanical energy, with high efficiency, the turbine should preferably be driven at high speeds. It is for this reason that the turbo-generator employed is usually a high speed rotary machine with two- or four-poles rotating at 3000 rpm or 1500 rpm for 50 Hz 3600 rpm or 1800 rpm for 60 Hz. Such a high speed generator cannot avoid a great centrifugal force developed in the rotor. For this reason, it is more preferable for the generator to be longer in its axial length and smaller in its diameter. The rotor is thus constructed as a nonsalient pole type mounted on a cylindrical forged shaft. Of course, the axial length of the rotor is limited because if it is excessively narrow and/or long, the shaft tends to greatly bend, causing a great vibration of the machine. Accordingly, the axial length must be selected suitably. Though such is the case, the entire axial length of the generator system including a single high pressure turbine and two low pressure turbines comes up to 50 m for a system of 500 to 600 MW and 65 m for a system of 1,100 to 1,200 MW.
Referring now to FIG. 1, there is shown an outer appearance of a turbo-generator coupled to a turbine system including a section A for a high pressure turbine, and a section B for three low pressure turbines, thus forming a so-called four-chamber turbine. A section C indicates a generator of which the rotary shaft is directly coupled with the shaft of the turbine system. A section D indicates an exciter section such an exciter, serving as an auxiliary machine of the generator. As mentioned above, the turbo-generator has a long axial length and is driven by high temperature steam of 500.degree. C. or more. It therefore suffers from a large thermal expansion. Particularly, since the rotary shafts of the turbine section and the generator section are directly coupled to each other, the thermal expansion of the rotary shaft of the turbine section affects the generator section. For this, the rotor of the generator is installed with precautions to compensate for a thermal expansion of the rotary shaft of the system.
FIGS. 2A, 2B and 2C show the relative axial locations of the stator and the rotor at three different operating states. Generally, in an electric rotary machine, the core lengths of the rotor and stator are determined in consideration of the respective magnetic loadings and the electric loadings. In the conventional rotary machine, the length L.sub.c of the stator core 1 is determined to be substantially equal to the effective length L.sub.f of the magnetic pole of the rotor 2 (referred to as a rotor effective length hereinafter).
As described above, the turbo-generator is installed by taking into consideration the thermal expansion of the rotor. In a conventional turbo-generator, therefore, the axial center of the rotor core 2 is shifted with respect to the axial center of the stator core 1 at no load condition. That is, the rotor is driven at no load in such a state that the rotor effective portion is projecting by .delta..sub.a from the stator core toward the turbine side, as shown in FIG. 2A. When the generator is driven at a rated output state, the rotor is subjected to the thermal extension of the turbine rotor and moved to a position where the axial center of the rotor 2 is substantially coincident with the axial center of the stator core 1, as shown in FIG. 2B. As a result, the offset as indicated by .delta..sub.a in FIG. 2A disappears. Also, it is possible that due to sudden change of the generator load or other possible conditions the rotor shaft moves and, in some cases, the rotor effective portion projects over the corresponding end of the stator core at the side remote from the turbine, as indicated by .delta..sub.c in FIG. 2C. In this way, the generator rotor axially moves so that the rotor effective portion moves in its axial direction within a range defined by .delta..sub.a and .delta..sub.c shown in FIGS. 2A and 2C.
Magnetic fluxes due to the magnetomotive force developed by the armature winding of the stator and the field winding of the rotor appear at the stator core end portion during the normal operation under substantial load. The fluxes concentrate at the tips of the teeth of the stator core, resulting in local overheating of the portions. Particularly, the axially extending magnetic fluxes entering the stator core end faces are normal to the laminated flat steel sheets of the stator core, thereby causing eddy currents which accelerate the local overheat of the stator core. This may lead to the burning of the core or deterioration of the insulation of the stator windings, resulting in lower reliability of the turbo-generator and the associated apparatus. Particularly, the amount of the fluxes axially entering the core end are greatly dependent on the relative position of the rotor effective portion to the stator core or the offset values .delta..sub.a and .delta..sub.c shown in FIGS. 2A and 2C.
Turning now to FIG. 5, there is shown a temperature distribution along the axial direction of the stator core. A curve I shows a temperature distribution when the stator and rotor are positioned as shown in FIG. 2B. The curve I shows that the temperature is higher at the stator core end portion near to the allowable highest value. In the cases where either one end of the rotor effective portion projects outwardly over the stator core end face, as shown in FIGS. 2A and 2C, the amount of the magnetic fluxes entering the stator core end corresponding to the projecting rotor end increases thereby also increasing eddy currents, which in turn increase the thermal energy produced there so that the temperature at the stator core end abruptly rises, as depicted by the curve II. As a result, that portion of the core may be burned and the insulation deterioration of the stator windings at the portion of the stator core may be accelerated. Finally, it may lead to a serious accident preventing normal operations of the turbo-generator, with the result that the reliability of the generator is considerably lowered. A curve III shows a temperature distribution on the side where the rotor end is recessed from the stator core end.
Hitherto, various attempts have been proposed to prevent the stator core end portion from being excessively heated, as disclosed in U.S. Pat. Nos. 4,031,422 issued to A. F. Armor et al on June 21, 1977, entitled "Gas Cooled Flux Shield for Dynamoelectric Machine" and No. 3,731,127 issued to D. B. Harrington on May 1, 1973, entitled "Generator End Tooth Flux Shield" and Japanese patent application No. 52068/76 by Tokyo Shibaura Denki K.K., published before examination on Nov. 11, 1977 as Laying-open No. 135007/77 and entitled "Rotary Machine." In those prior art disclosures, shield plates are disposed near the end portion of the stator core to prevent the magnetic fluxes from entering into the stator core end. However, the shielding effects of such shield plates are usually limited and hence it is difficult to satisfactorily prevent the temperature rise at the stator core end in question.